As explained in, for example, T. Nakanishi et al., “Application of Work Roll Shift Mill—HCW Mill—to Hot Strip Plate Rolling”, Hitachi Review, Vol. 34 (1985), No. 4 and K. Eckelsbach, G. Kneppe, D. Rosenthal, H. Wolters “Schedule-Free Rolling Strategies Based on Contour Control for Flexible Hot Strip Mill Concepts”, SMS-Schloemann-Siemag AG, Düsseldorf and Hilchenbach/Germany, ISIDIM '97 Conference Proceedings, pages 163-171, wear on the work rolls has a significant effect on the contour of the roll gap. The roll-gap contour is also affected by the crowning of the work rolls, the so-called “thermal crowning”, which is a consequence of the heating-up which goes hand in hand with the operation of rolling. Both the wear on the rolls and the thermal crowning may become so severe that the roll-gap contour, and hence too the metal strip which passes through the roll gap, are subject to differences from the desired shape which go beyond a size allowable as a tolerance.
To ensure that there is a proper roll-gap contour in spite of the differences in the shape of the work rolls which result from the wear on the rolls and the formation of the crowning, it is proposed in, for example, EP 0 276 743 B1 that the work rolls be shifted cyclically in opposite directions along their axes of rotation. Work rolls which were ground to an ideal cylindrical or crowned shape were used in this case.
As a result of this practical step, the wear on the rolls and the thermal crowning can be regularized to an appreciable degree, thus enabling the risk of so-called high spots or humps on the flat product being rolled to be appreciably reduced. In practice however this is not a successful way of completely avoiding the unwanted differences in shape in question.
It is also known, from DE 10 2004 031 354 A1, for intermediate or supporting rolls, which are provided in the manner described in for example DE 30 38 865 C1 with a so-called CVC grind and against which the work rolls of the given roll stand are supported, to be shifted axially in such a way that the roll-gap contour has in each case a parabolic component (a 2nd order component). At the ratios of work-roll length to work-roll diameter which exist in hot rolling mills, this component corresponds to the component represented by the bending of the work rolls. By means of variations in the bending force, a cyclic shift of the CVC rolls in opposite directions can then be performed to compensate for wear without this causing any change in the roll-gap contour. However, because, unlike direct shifting of the work rolls themselves, the shifting of the intermediate or supporting rolls calls for an appreciably larger range of adjustment if it is to affect the roll-gap contour, limits are set to this procedure in practice. In this way, steep transitions form between the region of the work roll which has been worn and its unworn region which result in what are termed tight edges on the rolled stock after a comparatively short time in operation. “Tight edges” of this kind produce edges which undulate in short cycles on the edge of the metal strip which is being rolled at the time. This may happen if the work rolls have to be shifted beyond the range of adjustment used for compensating for the wear on the work rolls. In the case of the rolls which are used in the known methods, the occurrence of this phenomenon generally makes it necessary for the rolls to be changed at an early point in time or results in marked restrictions on a schedule-free rolling program, i.e. a rolling program which is carried out without reference to the width of the strip which is to be rolled at the time.
Even if the prior art explained above assumes that the roll-gap contour can be reduced to, in essence, a parabola, i.e. a 2nd order function, the shape of the roll gap may however be composed in practice of, in addition, 4th order components and large components of even higher orders. This is particularly true when the roll pressures are high, such as they need to be when very small target thicknesses are being produced in the rolled stock. The 4th and higher order components affect the roll gap particularly when rolled stock has to be rolled whose width is large compared with the width of the roll gap, thus producing a ratio of the width of the work rolls to the width of the rolled stock which is adversely large. Precisely when rolled stock of large widths is being rolled, these conditions may result in considerable flaws in flatness because almost no flow of material transverse to the direction of rolling can take place in thin metal strip.
To minimize the effect of higher order components of shape, there have been proposed in EP 0 294 544 B1 and WO 03/022470 grinds for rolls which likewise affect components of the fourth and higher orders by axial shifting in opposite directions.
Also known, from EP 0 672 471 B1, are what are referred to as “anti-hump rolls” by whose axial shifting in opposite directions components of shape of higher orders can be acted on principally in the edge region of the roll-gap contour. If rolls of this kind were shifted cyclically relative to one another to correct for wear and thermal crowning, what would be produced as a consequence would be components of the fourth or higher orders which could not be compensated for by bending. There is also the risk of tight edges if the rolls are shifted after a long period of rolling. When rolls of this type are used, an even more severe limitation of the rolling schedule is therefore required.
Despite the large number of attempts which have been made to compensate for or eliminate the adverse effects of the change in the shape of the rolls of a roll stand as a result of wear and heating-up, it is only with difficulty that components of shape of the fourth or higher orders can be corrected for in practice at the roll-gap contour. This is particularly true on hot rolling lines, where the width of the material to be rolled is in each case wide in comparison with the width of the rolls. The outcome of this is that, even when use is made of specially ground rolls and of a cyclic shift of the rolls in opposite directions, it still has to be accepted that there will be non-flatness on the rolled stock obtained.